This application claims the priority benefit of Japanese application serial nos. 2001-083469, filed on Mar. 22, 2001, 2001-086532, filed on Mar. 26, 2001 and 2002-028623, filed on Feb. 5, 2002.
1. Field of Invention
The present invention relates to a photoreceptor that has a surface layer with a specific structure and thereby has high sensitivity, high resolution and high durability. Moreover, the present invention relates to an electrophotographic method, an electrophotographic apparatus and a process cartridge used in electrophotography that use the photoreceptor of this invention.
2. Description of Related Art
In the prior art, the electrophotographic methods, which include the Carlson method and various modifications thereof, are widely used in copying machines and printers. In the photoreceptor used in such electrophotographic methods, organic photosensitive materials have been used recently since they are cheap, pollution-free, and suitable for mass production. The types of organic-type electrophotographic photoreceptor include the photoconductive resin type represented by polyvinylcarbazole, the charge-transfer complex type represented by 2,4,7-trinitrofluoreone (PVK-TNF), the pigment dispersion type represent by phthalocyanine binder, and the function separated type using a charge-generating material and a charge-transferring material in combination. Among them, the function separated type gets much attention.
The forming mechanism of an electrostatic latent mage on a photoreceptor of function separated type is explained below. The photoreceptor is firstly charged and then irradiated with light, which passes through a transparent charge-conducting layer to be absorbed by a charge-generating material in a charge-generating layer. Charges will be generated from the charge-generating material due to the light absorption and then injected into the charge-transferring layer and driven by the electric field established by charging to move in the charge-transferring layer. The charges induced by the light absorption will neutralize some charges on the surface of the photoreceptor to form an electrostatic latent mage thereon. A charge-transferring material mainly absorbing UV light and a charge-generating material mainly absorbing visible light can be used together in a photoreceptor of function separation type, and such a combination is particularly advantageous.
Recently, the miniaturization of the photoreceptor is desired and the photoreceptor and the machine using the same are both required having a high durability (long lifetime). Accordingly, the organic photosensitive bodies have been developed energetically and are actually progressive in the issue of high sensitivity and high electrostatic durability of the photoreceptor. However, the mechanical durability of the photoreceptor, especially the wear resistance of the photoreceptor, is always considered to be insufficient. In view of this, the photosensitive bodies that have surfaces having improved mechanical durability are proposed, wherein those using binder resin on their surfaces are frequently investigated. However, those methods are always not satisfactory.
In another point of view, the durability problem can be solved by forming a protective layer on the outmost surface of the photoreceptor. The effects of using a protective layer as the surface layer of a photoreceptor, as started from the cases of the inorganic photosensitive bodies, are discussed in Japan Patent Publication No. Hei 2-3171, Hei 2-7058 and Hei 3-43618. Where a protective layer is disposed on the surface of an inorganic photoreceptor, the protective layer preferably comprises fillers having low specific resistance. Thus when the surface of the photoreceptor is being charged, the bulk of the protective layer and the photoreceptor/protective layer interface can also be charged easily. Certainly, another merit lies on that the latent image is not formed on the surface of the photoreceptor but in the protective layer (including the interface with the photoreceptor), and the shape (defects or the like) of the surface of the photoreceptor therefore has little affect on the latent image. However, if the protective layer serving as a surface layer is intended to have the aforementioned functions, a large amount of electrically conductive metal oxide fillers have to be added into it. In this situation, since the bulk or the surface of the surface layer has a low resistance, the so-called xe2x80x9cimage blurxe2x80x9d effect will occur as a drawback after repeated use even if the transparency of the surface layer is maintained. Japan Patent Publication No. Hei 2-7057 and JP-2675035 disclose a method for solving this problem, in which the concentration of the electrically conductive metal oxide in the surface layer varies along the vertical direction from the surface of the coating. By using this method, the drawbacks such as the image blur and the flowing defects can be prevented.
Moreover, as another method for solving the image blur problem with treatments during the process, an apparatus is used mounted with a drum heater for heating the photoreceptor. Though the image blur can be prevented from occurring by heating the photoreceptor, the incorporation of the drum heater will inevitably increase the size of the photoreceptor. Therefore, the method cannot be applied to miniaturized photosensitive bodies currently used in the mainstream accompanied with the miniaturization of electrophotographic apparatuses. In addition, high durability of a miniaturized photoreceptor is also hard to achieve by using this method. Moreover, the incorporation of the drum heater causes many problems in real use, such as the increased size of the photographic apparatus, the remarkably increased electricity consumption, and the increased time needed to setup the apparatus.
Because the transparency of the protective layer has to be maintained, it is important to assure that each component of the protective layer is transparent to the light used for writing the image. Particularly, the filler contained in the protective layer has a refractive index different from that of the binder resin used in the protective layer in most cases, so the protective layer tends to be opaque. To improve the problem for maintaining the transparency of the film, the filler can be made to have a size as small as possible. When the size of the filler is smaller than the wavelength of the light for writing the image, light scattering will not occur substantially and the protective layer becomes transparent. For example, a method is disclosed in which the mean size of the metal grains or the metal oxide grains contained in the filler is smaller than 0.3 xcexcm (Japanese Patent Application Laid Open No. Sho 57-30846), so that the protective layer becomes substantially transparent and the residual voltage accumulation can also be inhibited. Though its effect on maintaining the transparency of the protective layer is recognized, the use of such fillers has little effect on increasing the wear resistance of the protective layer. In view of this, instead of increasing the wear resistance of the filler, increasing the wear resistance of the binder is a more effective way for improving the wear resistance of the protective layer. Specifically, when an inorganic protective layer with durable electrostatic properties and high wear resistance fabricated by using this method is incorporated with an inorganic photoreceptor represented by the saline photoconductive layer, a photoreceptor having a quite long lifetime can be obtained. Moreover, the transparency of the protective layer can also be obtained by using fillers with a mean grain size larger than 0.3 xcexcm if only the dispersibility of the grain is high enough. On the other hand, if the fillers condense to a certain degree, the transparency of the protective layer will still be reduced even if the mean grain size is smaller than 0.3 xcexcm.
On another aspect, the photoreceptor has shifted toward the pollution-free organic photoreceptor and one can even say that all of the photosensitive bodies used in the world are of organic type. With the target of fabricating highly durable organic photosensitive bodies, the development of the protective layer capable of matching with the organic photosensitive layers is a major issue.
In view of this, for example, a method that uses a mixture of at least two kinds of fillers having different grain sizes or different specific weights on the outmost surface of the photoreceptor is described in Japanese Patent Application Laid Open No. Hei 8-234471 and Hei 8-314174. In this case, the aforementioned trade-off relationship is relaxed as compared with the cases where only one kind of filler is used, and the design margin thus can be increased. However, since the two kinds of fillers having different grain size must be densely packed to a certain degree in the protective layer to meet the requirement of wear resistance, the transparency of the coating film is reduced and the scattering of the writing light is increased with a degree dependent on the species of the filler. Therefore, such methods are always not satisfactory. Moreover, if the filler with small grain sizes (large surface area) is close to the surface of the protective layer, various materials like reactive gases will be easily adsorbed on their surfaces to easily cause degradation of the photoreceptor (e.g., image blur). Therefore, when two kinds of fillers having different mean grain sizes are used together, the filler with small grain sizes has to be excluded from the surface of the photoreceptor. This is just the strategy adopted by this invention that use a protective layer with a gradient grain size distribution to solve the aforementioned problem.
Moreover, the inorganic photoreceptor usually adopts a positively charging method regardless the existence of the protective layer. On the other hand, the charge-transferring materials developed to be used in the organic photoreceptor are divided into the hole conducting type and the electron conducting type, wherein those of hole conducting type has been developed to a practical level in real use. Therefore, in order to develop the effects of the organic photoconductor (OPC) to the maximal limits, the laminated photosensitive layers of function separated type are all of negatively charged type. On the other hand, the development of mono-layer type or reverse-layer type photosensitive layers is not in the mainstream.
One of the reasons that the protective layer techniques developed for the inorganic photoreceptor cannot be directly applied to the organic photoreceptor is that the charging types of the two are different (positive or negative). Since the absolute values of the charging voltages of the inorganic photosensitive body and the organic photosensitive body are almost the same, the positively charged one generally has a higher charging efficiency in consideration of the discharging efficiency of the discharger. In addition, by comparing the positively charged one and the negatively charged one, it is known that the amounts of the reactive gases or the like generated in the negatively charged one is much larger. It is also understood that at least the reduction of the surface resistance of the photoreceptor will causes the image blur problem, while the main cause of the reduction of the surface resistance is the adsorption of low-resistance materials on the surface of the photoreceptor that are generated due to the reactive gases.
In order to improve the problem, a contact-type charging method is proposed, in which a positive image data is disclosed certainly with a low ozone concentration near the charging member. However, according to the measuring results of the inventor, it is understood that the amount of the adsorbed low-resistance materials in the contact-type charging method is the same as that in the non-contact-type charging method. The reason is considered to be that the gap between the charging member and photoreceptor is so narrow that the gas flow cannot pass through it when the voltage applied to the charging member is being lowered for discharging. Another reason may be that the charging member is in contact with the photoreceptor so that the low-resistance materials is directly forced onto the surface of the photoreceptor.
So far, the method capable of simultaneously solving the aforementioned problems has not been found. When an outmost surface layer containing fillers, such as a protective layer, is formed on the photoreceptor to attain high durability, the image blur effect and the residual voltage rise effect both become more severe, and there are still many problems to be solved for attaining high image quality in practice. Moreover, if a drum heater is required to be used for reducing the aforementioned adverse effects, a high durability cannot be achieved for the miniaturized photosensitive bodies that need durability most, and the miniaturization of the apparatus and the reduction of the electricity are hindered in practice.
Moreover, the organic photoreceptor is superior than the inorganic photoreceptor in photosensitivity, spectral sensitivity range, pollution reduction and electrostatic durability, but the mechanical durability thereof has to be improved as soon as possible to develop its advantages. The development of the mechanical durability of the organic photoreceptor is most desired for designing a highly durably machine/process.
Full-color image forming apparatuses using electrophotographic methods can be divided into two types. The first type uses the so-called single method or single drum method. An electrophotographic photoreceptor and developing members of 4 colors are disposed in the apparatus. In this method, the toner images of 4 colors (cyan, magenta, yellow and black) are formed on the photoreceptor or a transfer member, which is a paper for output or an intermediate transfer body that transfers the image onto a paper once an image is transferred onto it. In this method, the charging member, the exposing member, the transferring member, the cleaning member and the fixing member disposed around the photoreceptor can be integrated. Consequently, the apparatus is smaller and has a lower cost as compared with the tandem method that will be described next.
The second type uses the so-called tandem method or tandem drum method. In this method, a plurality of electrophotographic photoreceptor is disposed in the apparatus. Generally, each drum is in combination with a charging member, an exposing member, a developing member and a cleaning member to form an electrophotographic element, which is duplicated into a plurality corresponding to the number of the drums (usually 4). In this method, the toner image of each color is formed on an individual electrophotographic element and then transferred onto a transfer body in turn to form a full-color image. By using this method, the first merit is that the high-speed image formation is possible since the toner image of each color can be formed simultaneously. Therefore, the processing time required for forming the image in this method is one fourth of that required in the single method, which means the speed of the image printing is higher by four times. The second merit is that all of the members used in the electrophotographic elements, including the photoreceptor, substantially have higher durability since one photoreceptor is used 4 times to form a full-color image in the single method but is used only once in the tandem method.
However, this method also has the demerits such as the increasing size of the whole apparatus and the high cost. In order to solve the problems, the photoreceptor and all the members around it have to be miniaturized correspondingly to reduce the size of each electrophotographic element, so as to have the benefits of reducing the size of the whole apparatus and reducing the cost of the used materials. Consequently, the cost of the whole apparatus can also be lowered. However, in company with compactness and miniaturization of the apparatus, the durability of all members of the electrophotographic element including the photoreceptor needs to be improved and there are still new issues to be studied.
Moreover, since the images on different image forming elements must be formed simultaneously in the tandem-type image forming method, it is an important issue to make all of the image forming elements have substantially the same properties at the beginning as well as after a long period of time (repeated use). The most critical factor about this issue is the electrostatic property of the photoreceptor in the image-forming element. When plural image-forming elements are used, the variations of the properties of the photosensitive bodies are large and the color balance of the output image is thus changed. Therefore, the color reproducibility of the input image is lowered as being a lethal problem of the full-color image forming apparatus.
Accordingly, this invention provides a photoreceptor capable of forming a stable high-quality image suitable for repeated use with high durability. Specifically, this invention provides a photoreceptor that has high durability and is capable of inhibiting residual voltage rise and occurrence of image blur that will otherwise deteriorate the image, and is thus capable of forming a high quality image steadily even after repeated use for long time. Moreover, this invention provides a photoreceptor that is used in an image forming apparatus utilizing the tandem method, and has good color reproducibility even after repeated use.
Moreover, this invention provides an electrophotographic method, an ectrophotographic apparatus and a process cartridge used in electrophotography that use the photoreceptor of this invention, wherein the used photosensitive bodies need not to be exchanged. By using them, high-speed printing is possible and miniaturization of the electrophotographic apparatus can be achieved in accompany with miniaturization of the photoreceptor, and high quality images can be formed steadily even after repeated use.
An electrophotographic photoreceptor disclosed in this invention comprises a photosensitive layer containing at least an organic charge-generating material and an organic charge-conducting material on a conductive support, and a surface layer. The surface layer containing at least two kinds of filler having different volume-averaged grain sizes, wherein the different kinds of filler may comprise the same material or different materials. The fillers in the photosensitive layer have a gradient size distribution where the grain size of the filler continuously increases from the side adjacent to the photosensitive layer toward the surface side. Moreover, the above-mentioned size distribution represents the mixing ratio of at least two kinds of filler having different grain sizes, which is the same in the description hereinafter. Moreover, when the protective layer comprises a single layer, the continuously increasing size distribution means that the mixing ratio of at least two kinds of filler changes continuously in the single-layer protective layer.
Another electrophotographic photoreceptor disclosed in this invention comprises a photosensitive layer containing an organic charge-generating material and an organic charge-transferring material on a conductive support, and a surface layer. The surface layer comprises at least two sub-layers containing fillers of different volume-averaged grain sizes, wherein the fillers of different volume-averaged grain sizes may comprise the same material or different materials. The fillers in the photosensitive layer have a grain size distribution that gradually increases from the side adjacent to the photosensitive layer toward the surface side. Moreover, when the protective layer comprises multiple layers, the gradually increasing size distribution means that the mixing ratio of at least two kinds of filler changes step-by-step in the multiple-layer protective layer.
In the prior art, for an electrophotographic photoreceptor comprising a protective layer and a photosensitive layer containing an organic charge-generating material and an organic charge-conducting material on a conductive support, 3 trade-off relationships are usually observes: 1) the residual voltage is reduced with increasing wear-resistance; 2) the image blur is compelled with increasing wear-resistance; and 3) the resolution is improved with increasing wear-resistance. However, when the used protective layer comprises at least two kinds of filler having different volume-averaged grain sizes with a gradient size distribution (the grain size distribution continuously increases from the side adjacent to the photosensitive layer toward the surface side), it is observed that the 3 trade-off relationships are cleared simultaneously. Therefore, a photoreceptor capable of forming high quality images for repeated use with high durability can be designed. Consequently, an electrophotographic method, an electrophotographic apparatus and a process cartridge used in electrophotography capable of forming high quality images for repeated use with high durability can also be designed.
Similarly, when the protective layer comprises at least two sub-layers containing fillers of different volume-averaged grain sizes with the sub-layer having a larger filler grain size at the outmost surface side and the sub-layer having a smaller filler grain size at the photosensitive layer side, the 3 trade-off relationships are cleared simultaneously. Therefore, a photoreceptor capable of forming high quality images for repeated use with high durability can be designed. Consequently, an electrophotographic method, an electrophotographic apparatus and a process cartridge used in electrophotography capable of forming high quality images for repeated use with high durability can also be designed.
Furthermore, since the protective layer comprises at least two sub-layers containing fillers of different volume-averaged grain sizes with the sub-layer having a larger filler grain size at the outmost surface side and the sub-layer having a smaller filler grain size at the photosensitive layer side, the differences between the electrostatic properties and between the wear resistance of different photosensitivity bodies in the image forming elements can be reduced. Consequently, a full-color image forming apparatus using the tandem method can be designed with good color reproducibility even after repeated use.
The electrophotographic photoreceptor of this invention are further explained in the following preferred embodiments in accompany with drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.